Generate more detailed version information.

Old output:
flashrom v0.9.7-r1716 on Linux 3.8.0-6-generic (x86_64)

New output with svn with local changes:
flashrom v0.9.7-r1716-dirty on Linux 3.8.0-6-generic (x86_64)
modified 2013-08-21T14:56:25+0200, available from svn://flashrom.org/flashrom/trunk

New output with git with local changes when URL is unavailable:
flashrom v0.9.7-r1716-50e143e-2-dirty on Linux 3.8.0-6-generic (x86_64)
modified 2013-08-21T14:34:40+0200

New output with git with local changes when URL is available:
flashrom v0.9.7-r1716-f6845c7-2 on Linux 3.8.0-6-generic (x86_64)
modified 2013-08-21T14:34:56+0200, available from git@github.com:stefanct/flashrom.git git

Signed-Off-By: David Hendricks <dhendrix@google.com>
Signed-off-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>